10 



PHYSIOLOGY OF NUTRITION. 



surface to the Carbon dioxide- containing air, and their green tissue 

 is kept extended by the special arrangement of the nerves. The 

 nerves also convey to the mesophyll the water and mineral sub- 

 stances necessary for the life and functions of the cells of the 

 chlorophyll-containing parenchyma. The arrangement of the 

 nerves in the leaf is of course very different in different plants, 

 and we may proceed as follows to obtain special information on 

 this subject. A leaf of Impatiens parviflora is laid in alcohol 

 till the chlorophyll is extracted. We then place the leaf for a 

 time in a solution of 5 parts of chloral hydrate in 2 parts of water. 

 By this means the leaf is rendered highly transparent, and por- 



U-f 



FIG. 3. Transverse section of part of a mature leaf of Trifolium pratense : o.s, upper 

 side, u.s, under side of the leaf ; o, epidermis ; sp, stoma; oca, crystal of Calcium oxalate 

 in the crystal sheath of the vascular bundle ; "hlz, wood of the vascular bundle ; g, vessels; 

 wb, soft bast ; bf, bast fibres. Magn. 300. (After H. de Vries.) 



tions of it may be submitted at once to microscopic examination. 

 In the mesophyll we perceive elongated cells, which contain 

 bundles of raphides (crystals of Calcium oxalate). The leaf is 

 traversed by a fairly strong median nerve, from which lateral 

 nerves of the first order run to the edge of the leaf, where, curv- 

 ing upwards in an arc, they join on to the next higher lateral 

 nerves of the first order. The nerves of the first order give off 

 nerves of the second order, these nerves of the third order, and so 

 on, so that a much divided network is formed, of which the 

 ultimate very fine branches in part end blindly in the meso- 

 phyll. 1 



